In order to decrease toxic component emission from a spark ignition type internal combustion engine provided with such a type of exhaust gas purifying device as a catalytic converter or a thermal reactor, it is a well known technique to control the ignition timing so that it is delayed to obtain an advance angle which is smaller than an optimum advance angle where a maximum combustion efficiency is attained. Due to the delayed ignition timing, so-called after-burning takes place in the exhaust system of the engine, so that the catalytic converter or the thermal reactor can easily attain an activated condition by which the toxic components are effectively purified. However, the delayed ignition timing adversely affects a stabilized idling operation of the engine, resulting in so-called engine stall after the first combustion cycle is attained, or in a low idling rotational speed of the engine.
In order to prevent such drawbacks from being generated, a system has been proposed wherein a choke breaker is provided with a vacuum line having an orifice therein, for controlling the rate of opening of a choke valve, to produce a rich air-fuel mixture after the first combustion cycle is attained. The rich air-fuel mixture serves to stabilize the idling operation of the engine on one hand. However, the rich air-fuel mixture is, on the other hand, apt to produce a large amount of carbon which is attached to spark plugs.